This invention relates generally to optical systems and, more particularly, to optical systems that are protected from intense optical transients by non-linear optical technologies.
Optical telescopes are used in research, commercial, and defense contexts for a variety of imaging and communications applications. Both imaging and non-imaging optical telescopes gather light within a field of view and focus it by means of optical elements onto a final focal plane where an optical sensor, such as a sensor array or a human eye, is placed. These features of conventional optical systems make them vulnerable to intense optical transients. For example, laser light that falls within the field of view of the telescope is gathered and focused by the optics of the telescope. The resulting concentration of optical energy can cause significant damage to components of the telescope, including the sensor array, or to other optical sensors, such as human eyes. For example, most remote sensing telescopes employ a sensor array positioned at the final focal plane, the position where the intensity of the incoming light is greatest. A laser beam of sufficient intensity within the field of view of the telescope can quickly overload and burn out such sensor array. In the case that an eye of a human observer is used directly as the optical sensor of the telescope, an unchecked intense optical transient could blind the observer.
Some of the traditional approaches to protecting optical systems from the threat of intense optical transients involve non-linear optical materials used as optical limiters to limit the optical intensity at the sensor array. In certain non-linear optical materials, the transmissivity of the material varies as a function of the intensity of the incident light. Certain non-linear optical materials are more transparent to low-intensity, ambient light but become more opaque as light intensity increases. These materials can be used to attenuate intense incident light. In other types of non-linear optical materials, the refractive index changes as a function of the light intensity. These materials can be used to defocus intense optical transients. Yet another class of non-linear optical materials has phase conjugating properties. These are traditionally gas cells which, when exposed to very intense optical radiation, both reflect and conjugate the optical phase of the incident light. To provide better response and greater protection, it is desirable to place the non-linear optical device where the intensity of the light is highly concentrated. Thus, the non-linear optical media is traditionally placed near a focal point such as just in front of the detector system.
Most of the optical telescopes used for remote sensing are of the aplanatic Cassegrain (Ritchey-Cretien or RC) design. These RC designs suffer from at least two principal problems when non-linear optical materials are used as protection against intense optical transients. First, these designs do not have a natural intermediate focal point. Therefore, additional relay imaging techniques are used to re-image the incoming light onto the final focal plane array. In many of the re-imaging approaches, the size of the protection device is roughly the same size as the focal plane array. Thus, in at least some systems, the non-linear optical device must be quite large. Second, although RC systems can be designed with shorter focal ratios (e.g., an f-number of F/3), RC systems used for remote sensing traditionally feature long focal ratios. For example, the Hubble Space Telescope, an orbital imaging satellite, features RC optics with an F/24 focal ratio. These slow optical systems result in large spot sizes and poor concentration of energy. The poor concentration of energy, in turn, results in a poor response of the non-linear optical materials which, in turn, decreases the degree of protection the non-linear material can provide against intense optical transients. In other words, at these low concentration ratios, the non-linear materials may not begin to “turn on” significantly until the laser intensity is already near or beyond the damage threshold for the optical sensor. Some of the more recent satellite-based telescopes feature a three mirror anastigmat (TMA) optical design. While markedly different from RC optical systems, these designs have the same disadvantages as RC optical systems when it comes to protection from intense optical transients.
Hence, there is a need for optical systems that facilitate improved performance of non-linear optical technologies for protection against intense optical transients. In addition, there is a need for optical systems which reduce the size of such non-linear optical devices.